The present invention relates to a bipolar type thin film semiconductor device using the SOI (Semiconductor on Insulator) technique.
FIG. 4 shows one conventional example (1979 International Electron Devices Meeting, Technical Digest, pp., 510-513), and FIG. 6 shows another conventional example (1985 International Electron Devices Meeting, Technical Digest, pp., 432-435).
The first conventional thin film device shown in FIG. 4 includes a p+ silicon substrate 21 serving as an emitter region, an n+ polycrystalline silicon film 22 serving as a base region, and a p-type polycrystalline silicon film 23 serving as a collector region. The base region 22 is highly doped that its impurity concentration is equal to or higher than about 10.sup.21 cm.sup.-3. The device of FIG. 4 further includes a silicon oxide film 24, an emitter electrode 25, a base electrode 26, and a collector electrode 27. In this way, the first conventional device is in the form of a laminate type pnp bipolar transistor.
FIG. 5 shows characteristics of emitter current Ie and collector current Ic versus the base width in the laminated bopolar transistor of FIG. 4. As is well known in the art, the base width is the minimum spacing between the collector and emitter regions which are separated from each other by the base region. In this transistor, the base width is the thickness of the n+ polycrystalline silicon film 22. As shown in FIG. 5, the emitter current Ie and collector current Ic of the laminated bipolar transistor vary in dependence on the film thickness of the n+ poly-crystalline silicon film. Especially, the collector current Ic is varied steeply at a high rate of change with respect to the base width.
In general, the emitter current Ie of a bipolar transistor is expressed by; EQU Ie=Ib+Ic (1)
where Ib is a base current. Grounded emitter current amplification factor .beta. and grounded base current amplification factor .alpha. are expressed as follows; EQU .beta.=Ic/Ib (2) EQU .alpha.=.beta./(1+.beta.)=Ic/Ie (3)
From the equation (3), the condition which must be satisfied in order to make the grounded emitter current amplification factor .beta. of the laminated bipolar transistor of FIG. 4 equal to about five (.beta.=5), is obtained as follows; EQU .alpha.=5/(1+5)=5/6=Ic/Ie (4)
Therefore, in order to satisfy this condition that Ic/Ie=5/6 with the characteristics of FIG. 5, it is necessary to make the base width, that is, the thickness of the n+ polycrystalline silicon film, equal to or smaller than several hundred angstroms. Even though the impurity concentration of the base region 22 is made higher than the level of 10.sup.21 cm.sup.-3, the base width must be made lower than several thousand angstroms in order to increase the grounded emitter current amplification factor .beta. to a sufficient level.
However, the fabrication process required for accurately controlling the thickness of the n+ polycrystalline silicon film 22 at the level of several hundreds or several thousand angstroms is complicated and causative of an increase in the fabrication cost of the device.
The second conventional thin film semiconductor device shown in FIG. 6 includes a glass substrate 31, a transparent electrode 32 consisting of an ITO (an alloy of indium oxide and tin oxide) film, formed on the glass substrate 31, and a laminate 30 of amorphous silicon. The amorphous silicon laminate 30 includes an n-type collector region 33 consisting of an n+ highly doped layer 33a and an n-type lightly doped layer 33b, a first i layer (intrinsic layer) 34, a p-type base region 35, a second i layer (intrinsic layer) 36, and an n+ emitter region 37. A top layer 38 is an emitter electrode of an Al film, and an electrode 39 is an collector electrode. Thus, the second conventional device is an npn bipolar transistor having a laminated structure.
FIG. 7 shows a relationship between the grounded emitter current amplification factor .beta. of the bipolar transistor of FIG. 6 and a thickness t of the first i layer 34 formed between the base and collector regions. As shown in FIG. 7, the grounded emitter current amplification factor .beta. of the second conventional device is strongly dependent on the thickness t of the first i layer 34. For example, the thickness t of the first i layer 34 must be controlled below about 5000 angstroms in order to make .beta. equal to about 5 (.beta.=5).
In this case, too, the fabrication process required for accurately controlling the thickness t of a the first i layer 34 below the above-mentioned value is complicated and causative of cost increase. Furthermore, the difference in height between the laminate structure 30 and the remaining top surface of the substrate of the device of FIG. 6 is so increased that it is difficult to integrate this thin film bipolar transistor with other bulk semiconductor components formed in a silicon semiconductor substrate into the form of a compact IC.